Back in 1999, a woman in California cleaned up rodent droppings in her home. Two weeks later, her liver started failing. Then she started to bleed internally — a hemorrhagic fever that would kill her. Eventually doctors found a new virus in her body, which very likely came from a rat.

A few years later, a man in Arizona went to the hospital. The skin on his legs was infected and dying. Doctors had to amputate. His diagnosis? A new kind of leprosy.

Over in the Midwest, the problem has been new tick-borne diseases, some deadly. And in New England, doctors are dealing with a disease that causes Lyme-like symptoms but is caused by a different bacteria.

The pattern continues across the country and across the world. A spike in new infectious diseases is the new normal.

Over the past 60 years, the number of new diseases cropping up in a decade has almost quadrupled. The number of outbreaks each year has more than tripled since 1980.

"We're in a hyperinfectious disease world," says epidemiologist Michael Osterholm, who directs the Center for Infectious Disease Research and Policy in Minneapolis.

More than a dozen new viruses and pathogenic bacteria have appeared in North America in the past 20 years. Europe has had at least 18. Asia 17. Africa seven.

OK. By now, you might be thinking: Aha! I know what it is! Scientists have gotten better at detecting diseases — and the media have gotten better at giving us the heebie-jeebies about them.

That is true, says Barbara Han, a disease ecologist at the Cary Institute of Ecosystem Studies in Millbrook, N.Y. But it's not the full picture.

"There's never going to be a perfect way for us to take into account sampling bias," Han says. "But even when we try to compensate for how much more information there is, we just have more diseases overall. The data are very convincing."

Several teams have quantified the global rise in infectious diseases by analyzing disease databases and reviewing studies. They used a few tricks to take into account the fact that over time, doctors and scientists have developed better tools for identifying pathogens.

But still, the studies found a surge in diseases. Old diseases that we thought were gone — like the plague — are returning. New diseases are spreading into new regions. And more dangerous strains of old diseases are cropping up more frequently. (Not to mention the rise in drug-resistant versions.)

The only trend that looks reassuring is that the number of cases in outbreaks per person has declined over the past few decades, researchers at Brown University reported in 2014.

So the big question is: Why? Why is this era of new diseases happening now?

"Well, we've been boiling the frog for a long time. Eventually, it's cooked," says Toph Allen, a data scientist with EcoHealth Alliance, a nonprofit that is trying to prevent pandemics by looking for diseases in wildlife.

Wait. We're boiling the frog? You mean, humans are responsible? Yes.

Many scientists say we, humans, are to blame for this new disease era. That we're responsible for turning harmless animal viruses into dangerous human viruses.

Over the next month, NPR's global health team is going to explore why this is the case. We'll look at where these diseases come from. How they're unleashed and how they spread.

And why this isn't just a problem for poor countries. In fact, one of the hot spots for rodent viruses could be right here, stateside, in Nebraska.

What do you want to know about pandemics? Share your questions by submitting themin our special tool here. Our global health team will answer some of them in an upcoming story.